With reference to prior art FIG. 1, an amplifier 10 is often used as the input buffer of the receiver to amplify an incoming differential signal 15. The role of a differential amplifier is to sense changes in its differential input signal while rejecting changes in its common mode, or average, input signal. Removing the common mode component allows the differential amplifier to support relatively large voltage swings and effectively reject noise appearing on both input signals of a differential circuit.
Transmission channels typically exhibit low-pass and other filter effects that distort the transmitted signal, resulting in transmission errors at the receiver. Receiver equalization is known in the art to boost the voltage sensitivity of the receiver to the attenuated frequencies of the transmitted signal, thereby flattening the frequency response of the communication channel.
The prior art teaches employing an amplifier with de-emphasis to achieve equalization by compensating for the low pass characteristics of the transmission channel. The prior art amplifier 10 illustrated in FIG. 1 employs de-emphasis to achieve receiver equalization.
With reference to FIG. 1, ignoring the body effect of the M1 20 and M2 25 transistors of the amplifier 10, the gain of the amplifier can be derived as:
                              H          ⁡                      (            s            )                          =                                            V              out                                      V              in                                =                                                    g                                  m                  ⁢                                                                          ⁢                  1                                            ⁢                              R                L                                                    1              +                                                g                                      m                    ⁢                                                                                  ⁢                    1                                                  ⁢                                                      R                    s                                                        2                    ⁢                                          (                                              1                        +                                                                              sR                            s                                                    ⁢                                                      C                            s                                                                                              )                                                                                                                              Equation        ⁢                                  ⁢        1            
Where gm1 is the transconductance of M1/M2. The impedance of capacitor (Cs) 35 decreases with an increase in frequency. As a result, the amplifier gain is higher at higher frequencies than at lower frequencies due to the less equivalent regeneration impedance of Rs 30 and Cs 35. At low frequency, Equation 1 can be re-written as:
                              H          ⁡                      (            s            )                          ≈                                            2              ⁢                              R                L                                                    R              s                                ⁢                                          ⁢                      (                                          If                ⁢                                                                  ⁢                                                      g                                          m                      ⁢                                                                                          ⁢                      1                                                        ·                                      R                    s                                                              >>              1                        )                                              Equation        ⁢                                  ⁢        2            
For a given power, the maximum gain of the amplifier that can be achieved is gm1·RL at high frequency and the minimum gain of the amplifier that can be achieved is
      2    ⁢          R      L            R    s  at low frequency.
As such, the prior art amplifier 10 de-emphasizes the low frequency content of the signal to achieve equalization at the receiver. As such, some power is wasted due to the de-emphasis of the signal.
In light of the above, a need exists in the art for an improved amplifier that is power efficient in achieving equalization.